1. Field of the Invention
This invention pertains to the general field of precision optical alignment and, in particular, to a high-speed angular monitoring metrology module with diminished mechanical drift and improved accuracy.
2. Description of the Prior Art
The accuracy and precision of sample metrology with standard optical instrumentation depends among other factors upon reducing measurement aberrations. In particular, the introduction of an angular tilt in a sample under test with respect to the optical axis of an optical metrology system is the most frequent error in sample positioning, which requires proper detection and correction. In automated microscopic systems, where many loose samples are characterized sequentially in trays in an unconstrained position, the implementation of sample tilt correction for high-precision measurements may seriously affect system efficiency and throughput.
The detection of minor sample-tilts in interferometric microscope systems, for example, requires the identification of corresponding changes in orientation of interferometric fringes. These fringes may be used to provide tilt correction on the order of about 10 to 15 wavelengths, depending on camera resolution.
Larger angular sample displacements, on the other hand, may require the use of a basic autocollimator, as shown in FIG. 1. This invention is directed only at a new procedure for correction of such large tilts.
In the implementation of a prior-art autocollimator 1, illustrated in FIG. 1, a beam from a light source 2 is partly reflected by a beamsplitter 4 and collimated by a lens 6 towards the sample 8 under test. The fraction of light reflected by the sample is focused by the lens onto the surface of a position-sensitive detector 10, for example a CCD-camera. When the sample is tilted by an angle α, the reflected portion of the beam is focused onto a spot 7 that is displaced from a pre-calibrated null-tilt position 9. The amount of displacement d is indicative of the sample-tilt.
The sample-tilt detection and correction step in existing interferometric microscopes typically precedes the sample characterization step. As illustrated in the microscope system 30 of FIG. 2A, the sample-tilt detection system makes use of a point aperture A by flipping it into the illumination leg 12 of the microscope to provide an effective point source of light under illumination I.
A fraction of the light from this point source is delivered to a test surface 22 as a collimated beam by an optical system 14 of the illumination leg through a mirror 16, a beamsplitter 18 and an interferometric objective 20. Another fraction of light is delivered via a beamsplitter 24 to a reference mirror 26.
The microscope's imaging leg 32, which is composed of the objective 20 and an imaging system 28, forms two images of the aperture on a CCD-camera 34, as shown in FIG. 2B. One image RI is formed in reflection off the mirror 26 and serves as a reference, while another image SI is produced in reflection off the sample 22.
The spot image SI moves with changes in the sample tilt with respect to the mirror 26. A comparison between the so-called null position 9 (defined as the position of the reference image RI) and the position of the image SI using available software algorithms allows the required correction of the sample tilt, which is achieved when the two images RI,SI coincide in the detector plane of the CCD-camera, as shown in FIG. 2C.
To perform a sample measurement under the illumination I, the aperture A is removed from the illumination leg 12 of the microscope, as shown in FIG. 2D, and the imaging system 28 in the metrology leg 32 is substituted by a conventional optical system 36 for imaging the sample surface onto the camera 34. A conventional scanner 38 is used for scanning interferometric measurements.
The sample's tilt detection and correction in non-interferometric microscopes is implemented in a similar way. As illustrated in FIG. 3, the use of a non-interferometric objective 42 would result in only one image of the point light source being detected by the camera 34. This image originates on the sample 22, and changes its position with the sample's tilt. Thus, the use of a non-interferometric objective requires a pre-calibration of the null position, after which the sample's tilt is corrected by re-orienting the sample so that the image spot coincides with the calibrated null position, as would be clear to one skilled in the art.
Several aspects of prior-art tilt detection and correction limit the degree to which they can be utilized practically in an optical system. First, the position of the aperture in the illumination leg of the microscope system is critical. If the aperture is not returned to the very same spot between measurements of multiple samples, the tilt correction process in non-interferometric microscopes introduces inaccuracies due to changes in calibration settings. Further, tilt correction followed by sample measurement requires change of optical systems in the metrology leg of the microscope as well as flipping the aperture in an out of its illumination leg, which is time consuming and creates mechanical vibrations that reduce the measurement accuracy of the system. Finally, the light intensities required to perform sample tilt correction and sample measurement are often different, and needed adjustments in the light source consume extra time.
To the extent that tilt measurement precedes surface metrology, these drawbacks are unavoidable as long as a mechanical aperture and an additional optical system are introduced in the illumination and metrology legs of the microscope, respectively. Thus, there remains a need for a robust, high-speed and low mechanical-drift microscope system for correcting large tilts that overcomes the limitations described above.